The universe is 13-20 billion years old depending on who you ask.
Earth has been here 5 billion.
That gives us 8-15 billion years of missing time.
Within 100 to 200 years mainstream science will know how to get to the stars so consider a race 1000 years ahead of us, or 1 million years ahead. It would be probably impossible for them to miss us since we probably stand out like a sore thumb.
Here, this the most comphrehensive presentation that I have found on it.
As I pointed out above, I don’t consider it an impossibility, I simply consider it very unlikely that an alien race has visited us, was discovered by the US Government, which has subsequently covered it up for decades.
The technological gap – as I noted before, and you alluded to – implied by relatively common long distance interstellar travel is simply so enormous (in terms of relative advancement, not temporally – a race 1000 years in advance of us from our current era of development is FAR more advanced than for example we are in comparison to our medieval counterparts of a thousand years ago) that chance discovery simply would be so unlikely as to be practically impossible. And a planned contact with just one government is almost equally unlikely, as I don’t see any possible reason for an alien civilization to reveal itself to a human government far in advance of when they would plan to reveal themselves to the entire human civilization.
And it would be their choice – any race that far in advance of us would treat us as superstitious primitives (and rightly so). Why would they consult with one of our primitive governments, and hide themselves from all other human beings, any more than we would sneak into a aboriginal village in Papua New Guinea, and ask the local elders advice about whether we should hide from the other natives?
